A method of searching a digital recorded tape for a block of information by first mathematically determining the present location of the tape on the spool, and the number of revolutions that must be turned, and then counting the number of revolutions turned at high speed to arrive at the desired location.
Digital recorded tape, because of its slow access time and low cost, is normally used for back-up storage and for archives, or to access large records if long access times can be tolerated. Worst case times of 40 to 60 seconds would be typical. A hard disk or floppy is used for records that must be accessed faster, but the cost per bit is considerably higher. The reason that the access time of digital recorded tape is so long is that the current implementations scan the digital recorded information for location marks. The location marks are examined in a sequential manner until the requested location is found. There is a limit to the speed at which the recorded tape in contact with the read head can be driven while still allowing the read head to actually read location marks on the tape. Small computers use disks rather than tape as the main system mass storage memory because of this slow speed and the resulting long access time.
The top speed at which the tape can be driven would be significantly increased if the recorded tape were removed from contact with the read head. In this case, the current position of the tape and the relative distance to reach the requested target data block and an estimate of the time it would take for the tape to reach the target data block would be needed. Using the method, the tape would be advanced at high speed, retracted within the tape cartridge, to an approximate position of the target data block requested, the tape would be placed in contact with the read head(s), the exact position of the tape would be determined, and the tape would continue to be driven forward to the target data block. This method may still not be fast enough. In the many cases, after the high speed positioning approximation process, the tape may be too far from the target location mark in either a forward or reverse direction. A learning process applied to the high speed approximation process will correct most of these deficiencies.
U.S. Pat. Nos. 4,125,881, 4,398,300 and 5,179,479 describe similar systems and are incorporated herein by reference. Mathematical equations are used to predict on a dead reckoning basis when the tape is approaching the desired target, based on the encoded motion and the thickness of the tape on the encoded reels. However, the accuracy of these systems is not sufficient to stop the tape within a suitable distance of the desired target.
Japanese Publication No. 61-85688 to Matosudaas understood, describes a system where a Ram 5 contains a table of music recording numbers and a corresponding table of counter counted values. For each selection, the count of the target counter value is stored in RAM 41, the tape starts at the beginning and advances at high speed, while counting encoder pulses until the target counter count is reached. No computation of reel rotations is described.
In order for the access time of digital recorded tape drives to improve to the point where they are competitive with disk storage systems, a considerable improvement in the worst case access time would have to be accomplished.